Isobutanol is also referred to as 2-methylpropan-1-ol or 2-methylpropyl alcohol, and it is extensively utilized in the chemical engineering field. Isobutanol has heretofore been produced via a so-called fermentation process. For example, Patent Document 1 discloses a recombinant microorganism prepared by substituting at least one of the genes associated with the isobutanol biosynthesis pathway with a gene originating from an organism other than the host microorganism and a method for producing isobutanol utilizing such recombinant microorganism. Genes introduced into host microorganisms are the acetolactate synthase gene, the acetohydroxy acid reductoisomerase gene, the acetohydroxy acid dehydratase gene, the branched chain keto acid decarboxylase gene, and the branched chain alcohol dehydrogenase gene. Examples of host microorganisms that are employed include Escherichia coli, Bacillus subtilis, Saccharomyces cerevisiae, and Lactobacillus plantarum. 
Patent Document 2 discloses a recombinant microorganism with a metabolic pathway modified so as to be capable of producing an alcohol having 3 to 5 carbon atoms. Patent Document 2 also discloses the introduction of a mutant ketol acid reductoisomerase gene resulting from an increase in NADH dependence or modification of coenzyme specificity from NADPH specificity to NADP specificity into such recombinant microorganism. Further, Patent Document 3 discloses a mutant ketol acid reductoisomerase gene capable of binding to NADH. Techniques of both Patent Documents 2 and 3 are intended to increase NADPH and to decrease NADH in the isobutanol biosynthesis pathway, thereby improving the isobutanol yield.
None of Patent Documents 1 to 3 are sufficient in terms of the isobutanol yield, and further improvement in the isobutanol yield has been desired in the production of isobutanol via a fermentation process.